Obama returns to Prague, where a year ago he unveiled his vision for a world without nuclear weapons, to sign the arms reduction treaty that still requires ratification by both U.S. and Russia.

Reporting from Prague, Czech Republic — With an agreement to scale back the weaponry of the world's two greatest nuclear powers, President Obama and Russian President Dimitry Medvedev signed a long-sought treaty that still will require the ratification of both governments.

One year after unveiling his vision here for a world without nuclear weapons, Obama returned this morning to sign a treaty with the Russian president that both sides call a major step forward on worldwide arms control.

In a ceremony at the medieval Prague Castle, Obama and Medvedev signed a "New START" treaty that administration officials say will bring U.S. and Russian nuclear arsenals to their lowest levels since the early 1960s.

The White House says the treaty will reduce the number of long-range deployed nuclear warheads by 30%, while also taking the two nations several strides forward in overall relations.

"One year ago this week," Obama said after the signing, "I came here to Prague and gave a speech outlining America's comprehensive commitment to stopping the spread of nuclear weapons, and seeking the ultimate goal of a world without them. I said then -- and I will repeat now -- that this is a long-term goal, one that may not even be reached in my lifetime.

"But I believed then -- as I do now -- that the pursuit of that goal will move us further beyond the Cold War, strengthen the global nonproliferation regime, and make the United States, and the world, safer and more secure," he said.

Medvedev, who sat by Obama's side for the signing in an ornate hall of Prague Castle, said: "Here in this room a truly historic event took place. . . . I believe that this signature . . . will create safer conditions for life here and throughout the world.

"Just a couple of months ago, [this] looked like mission impossible," Medvedev added, but now, "this is a win-win situation. No one stands to lose from this agreement. . . . The entire world community has won."

But there were other tough subjects on the table as Obama and Medvedev convened for a one-on-one meeting at the castle, and then for the signing ceremony and a brief news conference.

Obama and Medvedev were expected to discuss the thorny issue of sanctions against Iran to curb its nuclear program. While Russia has signaled openness to the idea of supporting sanctions by the United Nations Security Council, Medvedev has not clearly stated how deeply committed he is to the actual imposition of sanctions.

An administration official said shortly before the bilateral meeting this morning that he did not expect any major "pronouncements" on that topic coming out of the meeting.

"My expectation is we are going to be able to secure strong tough sanctions on Iran this spring," Obama said after the treaty signing. "President Medvedev and I have been able to build up a level of trust . . . that helped to facilitate our ability to work together jointly to present to Iran reasonable options that would allow it to clearly distance itself from nuclear weapons and pursue a path of peaceful nuclear energy."

Medvedev said that any sanctions should be designed to achieve a goal, not simply to punish Iran.

"Those sanctions should be smart, sanctions that are capable of producing proper behavior," Medvedev said. "I have outlined our limits for such sanctions. . . . I, as president of the Russian Federation . . . will proceed from two premises: We need Iran to behave properly . . . and to maintain the national interest of our countries . . . smart sanctions should be able to motivate certain parties to behavior properly."

As Obama officials herald the new treaty as a sign of dramatic progress in U.S.-Russian relations, Russian officials have been issuing warnings that they could pull out of the agreement at some future date. Foreign Affairs Minister Sergei Lavrov has publicly emphasized that Russian adherence to the treaty's terms is linked to how the U.S. acts on its missile defense program.

In a blog post on the White House website this morning, the White House's new point man on the ratification effort downplayed the warnings, suggesting that they are less significant than the actual language of the hard-wrought treaty.

"Most treaties have a simple withdrawal clause, allowing a country to exit the particular treaty for any reason or no reason," wrote Brian McKeon, senior advisor to the National Security Council and deputy national security advisor to Vice President Joe Biden. "The withdrawal clause in the New START Treaty has a higher bar; it gives a party the right to withdraw if it decides that "extraordinary events" related to the treaty have "jeopardized its supreme interests."

"The Russian statement does no more than give the United States fair notice that it may decide to pull out of the New START Treaty if Russia believes our missile defense system affects strategic stability," McKeon said.